1. Field of the Invention
The present invention relates to a hydraulic power unit (HPU). More specifically, the present invention relates to an electrically powered HPU having a hydraulically operated failsafe mechanism. In one illustrative embodiment, the present invention is directed to a subsea HPU.
2. Description of the Related Art
A typical subsea wellhead control system, shown schematically in FIG. 1, includes a subsea tree 40 and tubing hanger 50. A high-pressure hydraulic line 26 runs downhole to a surface-controlled subsea safety valve (SCSSV) actuator 46, which actuates an SCSSV. A subsea control module (SCM) 10 is disposed on or near the tree 40. The SCM includes an electrical controller 12, which communicates with a rig or vessel at the surface 32 via electrical umbilical 30.
Through control line 22, the controller 12 controls a solenoid valve 20, which in turn controls the flow of high-pressure hydraulic fluid from hydraulic umbilical 28 to hydraulic line 26, and thus to SCSSV actuator 46. When controller 12 energizes solenoid valve 20, high-pressure hydraulic fluid from umbilical 28 flows through valve 20 and line 26 to energize SCSSV actuator 46 and open the SCSSV. The required pressure for the high-pressure system depends on a number of factors, and can range from 5000 to 17,500 psi. In order to operate the SCSSV, the hydraulic fluid pressure must be sufficient to overcome the working pressure of the well, plus the hydrostatic head pressure.
When solenoid valve 20 is de-energized, either intentionally or due to a system failure, a spring in valve 20 returns the valve to a standby position, wherein line 26 no longer communicates with umbilical 28, and is instead vented to the sea through vent line 24. The SCSSV actuator is de-energized, and the SCSSV closes. Typically, solenoid valves such as 20 are relatively large, complex, and expensive devices. Each such valve may include ten or more extremely small-bore check valves, which are easily damaged or clogged with debris.
Through control line 23, the controller 12 controls a number of solenoid valves such as 14, which in turn control the flow of low-pressure hydraulic fluid from hydraulic umbilical 16 to hydraulic line 44, and thus to actuator 42. Typically the low-pressure system will operate at around 3000 psi. Actuator 42 may control any of a number of hydraulic functions on the tree or well, including operation of the production flow valves. A typical SCM may include 10 to 20 low-pressure solenoid valves such as 14.
For economic and technical reasons well known in the industry, in subsea wells it is desirable to eliminate the need for hydraulic umbilicals extending from the surface to the well. Referring to FIG. 2, one known method for accomplishing this is to provide a source of pressurized hydraulic fluid locally at the well. Such a system includes an SCM essentially similar to that shown in FIG. 1. However, in the system of FIG. 2, high and low-pressure hydraulic fluid is provided by independent subsea-deployed pumping systems.
A storage reservoir 64 is provided at or near the tree, and is maintained at ambient hydrostatic pressure via vent 66. Low-pressure hydraulic fluid is provided to solenoid valves 14 through line 60 from a low-pressure accumulator 74, which is charged by pump 70 using fluid from storage reservoir 64. Pump 70 is driven by electric motor 72, which may be controlled and powered from the surface or locally by a local controller and batteries. The pressure in line 60 may be monitored by a pressure transducer 76 and fed back to the motor controller. Hydraulic fluid, which is vented from actuators such as 42, is returned to storage reservoir 64 via line 62. High-pressure hydraulic fluid is provided to solenoid valve 20 through line 68 from a high-pressure accumulator 84, which is charged by pump 80 using fluid from storage reservoir 64. Pump 80 is driven by electric motor 82, which may be controlled and powered from the surface or locally by a local controller and batteries. The pressure in line 68 may be monitored by a pressure transducer 86, and the pressure information fed back to the motor controller.
Subsea systems have also been developed which replace all the low-pressure hydraulic actuators 42 with electrically powered actuators, thus eliminating the entire low-pressure hydraulic system. One possible solution for eliminating the high pressure hydraulic system is to omit the SCSSV from the system, thus eliminating the need for high-pressure hydraulic power. However, SCSSV's are required equipment in many locations, and thus cannot be omitted from all systems. Also, because of the harsh downhole environment, it is not practical to replace the hydraulic SCSSV actuators with less robust electric actuators. Although the high-pressure hydraulic system remains necessary in may systems, it would still be desirable to reduce the number and/or complexity of the components which make up the high-pressure system.
The present invention is directed to an apparatus for solving, or at least reducing the effects of, some or all of the aforementioned problems.